I guess it's different for different people, but at least in severe cases weed causes a lot of the symptoms that anxiety causes. Makes you paranoid and overthink about what's going on around you as well as inside your own body. But I guess no two people are alike so 2 people with the same overall problem could have the same drug but have different responses/reactions to it.

edit: but I actually know people who developed panic disorder and general anxiety disorder FROM smoking weed.

That being said, you're spot on with the observation that it was his anxiety asking for a bus. Seems like he needs CBT (Cognitive behavioral therapy) along with meditation to learn to separate himself from his anxiety thoughts. Medication is just a patch, therapy is more important. I took CBT and it took me out of the hole.